The present invention relats to a process and a device for treating fumes issued from a catalytic cracker.
Fumes known as regeneration fumes (or effluents) issued from the catalysts used are produced during the catalytic cracking of hydrocarbon charges. These fumes contain pollutants, notably sulfur-containing (SO.sub.2, SO.sub.3, H.sub.2 S) and nitrogen-containing compounds and/or nitrates which have to be reduced or even totally removed because these pollutants are harmful to environment.
Numerous treatments have already been proposed for depolluting these fumes, notably for desulfurizing those effluents. A conventional way of treating such fumes consists in washing them after passage through heat exchangers. This technology requires heavy equipments and therefore represents a costly investment.
Another way of desulfurizing effluents issued from catalytic crackers consists no longer in utilizing a wet process, as described above, but in using a dry process, i.e. utilizing beds known as "circulating" beds. Patent applications FR-A-2,632,872 and FR-A-2,642,663 filed by the applicant show two designs based on the principle of the desulfurization of gaseous effluents in a circulating fluidized bed. The first application provides for the use of non regenerable absorbents such as limes or limestones; whereas the second application (FR-A-2,642,663) discloses the introduction of a regenerable absorbent in and absorption zone. A further step of regeneration of the used absorbent is then required, as well as steps of separation and recycling of the particles in the desulfurization zone.
In both of the above cited cases, the grain size of the absorbent mass ranges between 5 and 5000 micrometers and is preferably more than 100 .mu.m.
This grain size requires the use of large-sized cyclones downstream from the treatment zone.
Besides, with such grain sizes, a minimum temperature of about 850.degree. C. is required to obtain a sufficient decarbonatation velocity and efficiency.
Moreover, the thermal inertia of these reactors is considerable on account of the use of refractory materials.
Desulfurizing boilers known as "entrained bed boilers", i.e. with a pulverulent injection of the absorbent in the desulfurization zone, are more and more often used because, in relation to conventional desulfurizing boilers, such as those described above, such heat generators are less costly and easier to implement.
Patent applications FR-A-2,636,720 and EN. 90/08,311 filed by the applicant such prior art devices.